A few questions...
 

How do you make the user-made levels work?

AND

How do you get the editor? I have JJ2 with no special sub names and I don't have the editor, and it says you need 'the registered' version, how do you register? (the game I mean)

Well you must have the demo version. That does not include the editor and cannot (normally) play custom levels.

The full version is probably hard to get. I know there are some people here selling TSF (Jazz 2 The Secret Files), and that is a full version but its online play is incompatible with version 1.23.

It might be possible to play homecooked levels in the demo version by renaming them with official filenames and playing them that way. (Making backups of the old ones, of course)
But other then that, the level editor and homecooked levels don't work with the demo.

Re: A few questions...
 

"Home cooked"-levels will work with the registered version of JJ2. The level editor is also provided with the registered version.
The registered version of JJ2 is available through your local retailer, though nowadays it can be quite a quest to find a copy. Some webstores still cary the title or you could buy a second hand copy from Ebay.

Good luck! :)